1. Field of the Invention
This invention relates broadly to integrated electronic circuits, and more specifically to FET circuits for controlling voltages that exceed individual FET breakdown voltages.
2. Related Art
Wireless communications devices, especially handheld devices, are undergoing sustained development. Perhaps more than any other factor, the extreme popularity of cellular mobile telephones has motivated improvements in efficiency, speed, and cost-effectiveness for RF transmission circuits in handheld devices. Enhancing the efficiency of such circuits is highly desirable so that the size of the required batteries may be reduced, while their life is extended. Cost-effectiveness is clearly always desirable for consumer products, particularly when such products require periodic replacement to stay abreast of changes in the technology. Additionally, transmitters must meet stringent emission limits, which have been established in order to facilitate high communication density at minimal power levels.
A transmission Power Amplifier (PA) is a subcircuit found in most mobile wireless transmitters (e.g., cellular telephones). The PA subcircuit is typically the most power-consuming portion of such transmitters, and, also typically, is the source of the most significant unintended or “spurious” emissions. In order to extend battery life, to meet stringent spurious emissions standards, and to minimize the cost of these high-volume consumer items, there is a need to improve the speed and efficiency, while reducing spurious emissions and manufacturing costs, for such PA subcircuits.
A range of PA topologies have been developed, each having different advantages. For example, PAs of class A, B, C, D, E and F are well known in the art. PAs of classes A–C are designed to permit power dissipation in amplifying devices that operate in a linear region, which is to say that such devices intentionally conduct current while voltage is present across the device.
PAs of classes D, E and F attempt to reduce the power loss caused by such linear operation by employing amplifier devices as switches, rather than as linear amplifiers. However, the pulse-type outputs from such amplifiers generally require extensive filtering in order to establish a narrow-band sinusoidal output, as is typically required. While normal operation of class D–F PAs does not intentionally cause drive element devices to conduct while voltage is present across the devices, even switched devices consume real power due to current flowing while voltage is present during finite switching periods. Moreover, compared to drive devices in analog PAs operating at the same transmission center frequency, drive devices in class D–F switching circuits must often operate at much higher frequencies. The higher frequency signals include significant energy at undesired frequencies, and such undesired signal energies not only consume circuit power, but also require filtering to meet emission limits.
Integration of devices is generally desirable in order to improve various features of the resulting product, such as operating frequency and reliability, and may also reduce overall manufacturing costs. Field Effect Transistors (FETs) are extremely popular for both linear amplification and switching purposes in integrated circuits. However, integrated circuit (IC) FETs have a limited capability to withstand voltage between any two nodes, including gate-source, gate-drain, and drain-source node pairs. Such voltage withstand limitations may particularly impair the usefulness of IC FETs in high power switching circuits, in which inductive voltages may greatly exceed the supply voltage. As a particular example, the transmission output power capability of an RF PA is highly dependent upon the amplitude of the output voltage. One of the difficulties with existing PA technologies is that many otherwise desirably high-speed devices are fabricated using processes that tend to yield FETs having relatively low breakdown voltages. It is very desirable to solve this problem ant thereby provide a wider voltage range while retaining other desirable integrated device features. Such a solution enables integration on monolithic integrated circuits of power and control features that previously required separate processing, such as PA features and RF switch features. Integration of interacting circuits that were previously discrete will enhance yield and predictability, due to the process matching that is inherent in monolithic integration.
A circuit and method is described herein that enables stacked integrated circuit FETs to control signals that substantially exceed the voltage withstand capability of individual FETs. Such a circuit or method may be used in wide range of applications, though it was developed and is described herein primarily in the context of RF PAs. FET stacks may be operated in either linear or switching modes, and are well suited for controlling conduction between two nodes in a circuit that may operate at high frequency. They are particularly well suited for use in integrated circuits, where considerations of fabrication efficiencies often dictate that all FETs have substantially similar, and thus similarly limited, voltage breakdown characteristics. In such integrated circuits, FET stacks enable a designer to take advantage of efficiencies that may accrue from operation at higher voltages, without sacrificing the fabrication efficiencies that follow from employing device fabrication parameters that are optimized for devices having a low voltage capability.